1. Field of the Invention
The present invention relates generally to a developing apparatus used for developing an electrostatic latent image formed on an image carrying member in an image forming apparatus such as a copying machine or a printer, and more particularly, to a developing apparatus for performing development upon conveying a developer to a developing area opposite to an image carrying member by a developer carrying member with the developer held on the surface of the developer carrying member as well as exerting an AC electric field between the developer carrying member and the image carrying member, which is characterized in that the conditions under which the AC electric field is exerted is suitably set, to obtain an image having a sufficient image density and being fine in texture.
2. Description of the Related Art
Conventionally, in an image forming apparatus such as a copying machine or a printer, various developing apparatuses have been employed in order to develop an electrostatic latent image formed on an image carrying member.
One example of such a developing apparatus is a developing apparatus as shown in FIG. 1. In the developing apparatus, a cylindrical developer carrying member 11 having a magnet roller 11a provided on its inner periphery is provided in the main body 10 of the apparatus so as to be opposite to an image carrying member 1, so that a developer 2 contained in the main body 10 of the apparatus is supplied to the surface of the developer carrying member 11 by a developer supplying member 12 such as a bucket roller. The developer 2 is held on the surface of the developer carrying member 11 by the magnetic force of the magnet roller 11a, the developer carrying member 11 is rotated to convey the developer 2, and the amount of the developer 2 conveyed to a developing area where the developer carrying member 11 and the image carrying member 1 are opposite to each other is controlled by a controlling member 13, after which the developer 2 is introduced into the developing area to develop an electrostatic latent image formed on the image carrying member 1.
In order to efficiently develop the electrostatic latent image formed on the image carrying member 1 as well as to prevent a toner image formed on the image carrying member 1 from being scrapped by a magnetic brush formed of the developer 2 in the above-mentioned developing apparatus, a developing apparatus so adapted as to perform development upon respectively applying a DC voltage and an AC voltage in a superimposed state to a developer carrying member 11 from a DC power supply 14 and an AC power supply 15 and exerting an electric field obtained by superimposing an AC electric field on an DC electric field on a developing area where the developer carrying member 11 and the image carrying member 1 are opposite to each other, as shown in FIG. 1, has been developed.
When development is thus performed upon exerting the electric field obtained by superimposing the AC electric field on the DC electric field on the developing area where the developer carrying member 11 and the image carrying member 1 are opposite to each other, to obtain a halftone image, however, the change in the density of the halftone image corresponding to the change in a surface potential of the image carrying member 1 corresponding to the halftone image is large in a part of the surface potential. Therefore, no good halftone image is obtained, and the texture of the formed image is not sufficient.
Therefore, in recent years, a developing apparatus so adapted as to perform development in such a manner that a first period (an exertion period) during which an AC electric field is formed between a developer carrying member and an image carrying member and a second period (a stop period) during which the formation of an AC electric field is stopped are alternately repeated, a pair of electric field components of the AC electric field which are opposite in direction are exerted in the first period, and the final electric field component of the AC electric field is always an electric field component for returning a developer to the developer carrying member has been proposed, as disclosed in Japanese Patent Publication No. 14706/1990 (U.S. Pat. No. 4,610,531).
In a case where the pair of electric field components of the AC electric field which are opposite in direction is exerted in the first period, and the final electric field component of the AC electric field is always the electric field component for returning the developer to the developer carrying member, however, the AC electric field is always stopped in a state where the developer is returned from the image carrying member to the developer carrying member while being moved between the image carrying member and the developer carrying member, after which the second period is started. Also in the second period, the developer is continuously returned to the developer carrying member. When development is performed in the above-mentioned manner, therefore, the developer is not sufficiently supplied to the image carrying member, so that the image density of a formed image is decreased. When development is performed at high speed, no image having a sufficient image density is obtained, and the texture of the formed image cannot be improved.